Method and system for measuring deflections of structural member at multiple locations using multiple antennae

ABSTRACT

Multiple deflections of a structural member (SM) can be measured at multiple locations thereof, which deflections can be used to monitor changes in stiffness over time, using at least one antenna to measure the resonant frequencies at different harmonic frequency numbers of a predetermined radio frequency spectrum. One antenna can include a plurality of coils providing a first coil region that has a maximum sensitive to a first resonant frequency harmonic number and another antenna can include a plurality of coils providing a second coil region that has a maximum sensitivity to a second resonant frequency harmonic number that is different from the first predetermined resonant frequency harmonic number. Using different harmonic resonant frequency numbers, deflections at multiple regions of the SM can be determined concurrently using a plurality of antennae.

This application is a continuation-in-part of application Ser. No.16/589,736 filed 1 Oct. 2019, to be issued as U.S. Pat. No. 10,892,558.

BACKGROUND

The disclosures of U.S. Pat. Nos. 10,674,954 and 10,641,664 are bothincorporated by reference herein in their entirety. In particular, thesepatents disclose that an in-dwelling strain sensor is not needed tomonitor strain applied to a structural member. Instead, these patentsdisclose using an antenna spaced from a structural member without theantenna making contact therewith. The '954 patent discloses a loadingdevice that applies loading a structural member (SM) in relation theantenna held spaced at a fixed distance from the SM.

But there still remains a need for a methodology and a device/system formeasuring deflections of the SM at multiple locations concurrently,which deflections can be used to measure the stiffness of the SM atmultiple locations thereof. The present invention addresses this need.

SUMMARY

One aspect of the present invention is an apparatus for measuringdeflections in a structural member (SM) at multiple locations as the SMundergoes a displacement as a load is applied to the SM. The SM can bemounted to a target object to be monitored, the SM undergoing thedisplacement as the target structural object undergoes a displacement ordeformation.

The apparatus includes a plurality of antennae, including a firstantenna and a second antenna, and a support or support structure, suchas a frame. Each of the plurality of antennae includes a plurality ofcoils. The first antenna includes a first coil region configured toprovide a maximum sensitivity to a first resonant frequency harmonicnumber within a predetermined radio frequency spectrum. The secondantenna includes a second coil region configured to provide a maximumsensitivity to a second resonant frequency harmonic number, which isdifferent from the first resonant frequency harmonic number, within thepredetermined radio frequency spectrum.

Each antenna is configured to induce, upon applying a first electricalsignal thereto, a magnetic or electromagnetic field in the SM and createa coupling of the magnetic or electromagnetic field between therespective antenna and the SM. In particular, the characteristics of themagnetic or electromagnetic field coupling between (a) the first antennaand the SM are associated with a distance between a first predeterminedregion of the SM and the first coil region and (b) the second antennaand the SM are associated with a distance between a second predeterminedregion of the SM and the second coil region. Each antenna outputs asecond electrical signal representing the characteristics of themagnetic or electromagnetic field coupling between the respectiveantenna and the SM. Each of the second electrical signals containsinformation for determining a respective deflection at a respectivepredetermined region of the SM for each of the plurality of antennae.

The support structure can maintain the first coil region disposedconfronting the first predetermined region of the SM to be measured at afirst distance from the SM and the second coil region disposedconfronting the second predetermined region of the SM to be measured,which is different from the first predetermined region, at a seconddistance from the SM. The first distance can be equal to or differentfrom the second distance, and vice versa.

Each antenna can be a dipole antenna that operates as a displacementsensor and comprises a first coaxial cable including a first end and asecond end. The first end of the first coaxial cable receives the firstelectrical signal, and the first end of the first coaxial cable outputsthe second electrical signal.

The dipole antenna can further include a second coaxial cable includinga third end and a fourth end. The first coaxial cable includes a firstshield and a first center conductor. The second coaxial cable includes asecond shield and a second center conductor. The first and secondshields are electrically connected. The first center conductor at thesecond end and the second center conductor at the third and fourth endscan terminate without any connection. A predetermined length of thefirst center conductor is exposed at the second end and a predeterminedlength of the second center conductor is exposed at the fourth end. Thesupport structure maintains the exposed first and second conductorssubstantially parallel to each other.

The plurality of coils of each antenna include a plurality of firstcoils of the first coaxial cable and a plurality of second coils of thesecond coaxial cable. The support structure maintains the plurality offirst coils in a coiled configuration between the first end and thesecond end and the plurality of second coils in a coiled configurationbetween the third end and the fourth end.

Another aspect of the present invention is a monitoring system formonitoring changes in the SM at multiple locations as the SM undergoes adisplacement as a load is applied to the SM.

The system includes the plurality of antennae and the support structuredescribed above, and a load applying mechanism that can apply a load tothe SM. Each antenna can output the second electrical signal each timethe load applying mechanism applies the load to the SM. Again, thesecond electrical signals contain information for determining respectivedeflections at respective predetermined regions of the SM.

The system further includes a controller and at least one hardwareinterface. The controller includes a memory storing instructions and aprocessor configured to implement instructions stored in the memory andexecute a plurality of tasks. The at least one hardware interface isconfigured to receive the second electrical signal from each antenna,and convert the second signal that is readable by the controller. Theplurality of tasks include a first determining task and a storing task.The first determining task, based on the converted second electricalsignals of the first and second antennae from the at least one hardwareinterface, determines the characteristics of the magnetic orelectromagnetic field coupling between the first and second antenna andthe SM respectively at the predetermined regions of the SM, anddetermines the deflections at the predetermined regions of the SM. Thestoring task stores the determined deflections in the storage deviceeach time the load applying mechanism applies a load to the SM. Theapplied load values also can be stored concurrently or prestored.

The at least one hardware interface can be a network analyzer providedwith a plurality of ports or a plurality of network analyzers. The atleast one network analyzer or the plurality of network analyzers eachare configured to output the first electrical signal to each antenna,receive the second electrical signal from each antenna, and alsodetermine the characteristics of the magnetic or electromagnetic fieldcoupling between each antenna and the SM based on the receivedrespective second electrical signal.

The first determining task determines the deflections at the first andsecond predetermined regions of the SM at a predetermined interval foran evaluation period and the storing task stores the determineddeflections at the predetermined interval for the evaluation period.

The plurality of tasks can include a second determining task thatdetermines a shift in characteristics of the magnetic or electromagneticfield coupling between each antenna and the SM over the evaluationperiod.

The plurality of tasks can include a third determining task thatdetermines a temporal change in relative displacement of the SM at thepredetermined regions of the SM over the evaluation period based on theshift determined in the second determining task. The temporal change inrelative displacement of the SM at the predetermined regions over theevaluation period can be represented as a slope of resonantfrequency/load at the respective first and second resonant frequencyharmonic numbers within the predetermined radio frequency spectrum.

The load applying mechanism can apply a predetermined load to the SM,and the first determining task analyzes the determined characteristicsin relation to the predetermined load applied to the SM by the loadapplying mechanism to determine the deflections at the predeterminedregions of the SM.

Another aspect of the present invention is a method of monitoringchanges in the SM at multiple locations as the SM undergoes adisplacement as a load is applied to the SM. The method comprises anantennae providing step of providing the plurality of antennae describedabove, an antennae disposing step, an inducing step, an outputting step,a first determining step, and a storing step.

The antennae disposing step disposes the first coil region confrontingthe first predetermined region of the SM to be monitored at the firstdistance from the SM and the second coil region disposed confronting thesecond predetermined region of the SM to be monitored, which isdifferent from the first predetermined region, at the second distancefrom the SM.

The inducing step induces, by applying the first electrical signal toeach antenna, the magnetic or electromagnetic field in the SM andcreates the coupling of the magnetic or electromagnetic field betweeneach antenna and the SM. The characteristics of the magnetic orelectromagnetic field coupling between the respective antenna and the SMare associated with a distance between a respective predetermined regionof the SM as previously described.

The outputting step outputs, by each antenna, the second electricalsignal representing the characteristics of the magnetic orelectromagnetic field coupling between the respective antenna and theSM. The first determining step determines, based on the obtained secondelectrical signal of the respective antenna, the respective deflectionat the respective predetermined region of the SM. The storing stepstores the determined deflections at the first and second predeterminedregions of the SM in the storage device. The outputting step outputs thesecond electrical signal from each of the plurality of antenna to atleast one network analyzer provided with a plurality of ports or aplurality of network analyzers.

The method further can further include a repeating step of repeating theinducing step, the outputting step, the first determining step, and thestoring step at the predetermined interval for the evaluation period.

The method can further include a second determining step of determininga shift in characteristics of the magnetic or electromagnetic fieldcoupling between each antenna and the SM over the evaluation period. Themethod can further include a third determining step of determining atemporal change in relative displacement of the SM at the predeterminedregions of the SM over the evaluation period based on the shiftdetermined in the second determining step.

The method can further include a loading step of applying apredetermined load to the SM, and the first determining step analyzesthe determined characteristics in relation to the predetermined loadapplied to the SM in the loading step to determine the deflections atthe first and second predetermined regions of the SM.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an overall side perspective view of a loading deviceaccording to the present invention, with a structural member shown inphantom.

FIG. 2 illustrates a front view of the loading device.

FIG. 3 illustrates a top view of a distal frame (support structure) ofthe loading device.

FIG. 4 illustrates an enlarged view of an antenna configuration moreclearly illustrating the arrangement of the free end of the antenna, thecenter conductor of the coaxial cables are exposed.

FIG. 5 illustrates a bottom view of the distal frame.

FIG. 6 illustrates an alternative embodiment of a movable support thatincludes multiple lower loading members to provided multiple lowerloading points.

FIG. 7 schematically illustrates an embodiment of a system that canmonitor changes in the structural member.

FIG. 8 illustrates a controller or computer that can determine theinstantaneous or temporal change in stiffness of the structural member(over an evaluation period).

FIG. 9 illustrates an operational diagram, namely a flowchart of thepresent system for monitoring the stiffness in the structural memberbased on one sensor type (antenna).

FIG. 10 illustrates another operational diagram, namely a flowchart ofthe present system for monitoring the stiffness in the structural memberbased on another sensor type (displacement sensors).

FIG. 11 illustrates an example curve showing antenna resonant frequencyversus the applied load.

FIG. 12 illustrates slopes (mean and standard deviation) of the a linearsensor displacement versus the load curves for three compliance groups.

FIG. 13 illustrates sloped (mean and standard deviation) of thecorrected displacement versus the load curves for the different loadingpoint distances and for intramedullary rods representing an unhealedfracture.

FIG. 14 illustrates radiographs of the eight testing conditions of asheep tibia showing progressively destabilizing osteotomies.

FIGS. 15A-15B illustrate slopes (mean and standard deviation) of the (A)antenna resonant frequency versus load curves, and (B) corrected linearsensor displacement versus the load curves.

FIG. 16 is a schematic illustration of an antenna for detecting a SMdeflection in multiple discrete locations.

FIGS. 17A-17B schematically illustrate measuring deflection of a singleSM at multiple (three) locations using a single antenna.

FIGS. 17C-17E schematically illustrate measuring deflection of a singleSM at multiple (three) locations using multiple (three) antennae.

FIGS. 18A-18C illustrate, based on the embodiment of FIGS. 17A-17B,plots of the experimentally observed relationship between the antennasensitivity to the presence of a structural member (SM) at differentlocations on the antenna at the first, second, and fifth harmonics,respectively.

FIG. 19 illustrate plots representing the relative sensitivity to a SMdeflection based upon antenna location and resonant frequency harmonicnumbers.

FIG. 20 illustrates an embodiment of an antenna that can be used todetect a SM deflection at two locations.

FIG. 21 is a plot of the radio frequency spectrum for the antenna ofFIG. 20 where concurrent measurements were made at two locations,demonstrating the resonant radio frequencies of each harmonic shiftingindependently when a SM is displaced in the individual vicinitiesthereof.

FIGS. 22A-22B illustrate plots showing the amount resonant radiofrequency shift due to the displacement of a stainless steel SM, fromwhere a discrete location of the antenna making contact to 10 mmdisplacement therefrom, at two spaced antenna locations corresponding totwo positions of the SM to be monitored using the third and fifthharmonic frequencies, to demonstrate that the deflection of the SMcorresponding to one antenna location using one harmonic frequencyassociated therewith results in a minimal shift to the resonant radiofrequency at the harmonic associated with the other antenna location.

DETAILED DESCRIPTION

The present loading device and monitoring system can measure thedeflection of a structural member (SM) that deflects when a force isapplied, and thus measure the stiffness of the SM over time.

Referring to FIG. 1, the loading device LD includes a loading frameassembly FA, a driving (applying) mechanism DM, a (first) sensor S1, aplurality of (second) sensors S2, and a (third) sensor 3. The frameassembly FA can rest on a stable surface SS, such as a sturdy table or afloor, and include upper and lower loading members LMu, LMI that act asloading points on two sides of the SM, such as a fractured tibia fixedwith an intramedullary nail or other structure support, inmultiple-point bending configuration, such as a three-point bending(shown in FIG. 1) or four-point bending configuration (shown in FIG. 6).The locations of the loading points can be adjustable to accommodatevarious lengths of the SM. The loading members, including multiple upper(first) loading members LMu, that are positioned spaced from each otherat a desired distance and rigidly mounted to the frame assembly FA, andat least one lower (second) loading member LMI, which is driven towardor away (see the double arrows in FIG. 1) from the upper loading membersLMu using the driving mechanism DM secured to the frame assembly FA.

The driving mechanism DM can be a linear screw jack, which can bemanually controlled by a hand crank DM3 connected to a rotating shaftDM2. The sensor S1, which can be a load sensor, such as a load cell, canbe connected to a movable support or bed FA3 to measure the forceapplied by the driving mechanism DM.

The loading device LD in the present development uses two differenttypes of sensors, namely a first type that makes a physical contact withthe SM and/or material surrounding the SM, such as skin and tissue, anda second type that makes no physical contact with the SM or the materialsurrounding the SM. The stiffness of the SM can be determined from a(first) slope of a first deflection curve obtained from the plurality ofsensors S2, which is one of the two sensor types, versus an applied loadcurve, based on the driving mechanism DM applying different loads, whichis obtained from the sensor S1, and a (second) slope of a seconddeflection curve obtained from the sensor S3, which is the other of thetwo sensor types, versus the applied load curve. The second sensors S2measure the first deflection of the SM and the third sensor S3 measuresthe second deflection of the SM. Combining both types of sensors allowsmultiple measurements of stiffness, providing for a more robust andreliable measurement system.

The frame assembly FA provides a rigid platform on which the SM canrests, and has sufficient rigidity so that any deformation it mayundergo would be negligible versus the deflection that the SM undergoeswhen it is stressed. Specifically, the frame assembly FA includes a baseFA1 that is restable on a stable support SS, such as a floor or a table,a distal frame (support structure) FA2 that is held spaced from the baseFA1 at a predetermined distance, which can be adjustable, and a movablesupport FA3 that is configured to be movable between the base FA1 andthe distal frame FA2.

Referring to FIGS. 1-6, the base FA1 and the distal frame FA2 can bemade of steel, aluminum, rigid plastic, or carbon fiber material, or anycombination thereof. Specifically, in the illustrated embodiments, thedistal frame FA2 can be constructed from a pair of rigid main beams FA2a that are spaced apart parallel along a horizontal plane and secured inthat position using a plurality of support plates FA2 b and cross beamsFA2 c so that the main beams and the cross beams form a rigid structurethat is not prone to deflection when the SM undergoes stress. The mainand cross beams also can be stacked to provide an even more rigidplatform. The beams can include channels/slots for anchoring bolts andnuts, such as shown in FIGS. 1-3 and 5 for attaching accessories, suchas the second sensors 2 and the loading members LMu, LMI. The main andcross beams are commercially available. For example, heavy dutycommercially available T-Slot aluminum extrusion rails can be used, suchas available from McMaster-Carr(https://www.mcmaster.com/t-slotted-framing) or 80/20 Inc(https://8020.net/shop).). By loosening two outermost bolts B (see FIG.3), which secure the two support plates FA2 b fastened to the cross beamFA2 c to the main beams FA2 a, the respective cross beam FA2 c can bemoved in the longitudinal direction of the main beams FA2 a and securingthose bolts will maintain the cross beam in place. Moving the crossbeams also move the respective sensors S2 and the loading members LMumounted thereto. Accordingly, the mechanism for adjusting the sensorsand the loading members corresponds to the support plates FA2 b and thebolts securing the cross beams FA2 c to the main beams FA2 a.

Referring to FIGS. 1 and 5, the distal frame FA2 includes a plurality ofupper loading members LMu, each of which can be disposed along a lengthof the main beams FA2 a and secured to a desired position in relation tothe SM, to the cross beams FA2 c using fasteners. The upper loadingmembers LMu can be mounted to the cross beams FA2 c, as illustrated inFIGS. 1 and 5. Adding more cross beams FA2 c further increases therigidity of the distal frame FA2. Moreover, the position of the crossbeams FA2 c can be adjusted by loosening the bolts

Referring to FIGS. 1-2, the base FA1 can be made from an aluminum orsteel plates to rigidly mount the distal frame FA2, the movable supportFA3, guide rods FA4, and the driving mechanism DM. In this respect, thebase FA1 can be made from rigid material and construction that cansupport the movable support FA3 and the distal frame FA2, as well as thedriving mechanism DM. The frame assembly FA also includes at least apair of rigid guide rods or rails FA4 that space and maintains thedistal frame FA2 spaced from the base FA1 at a desired distance, whichcan be adjusted, for example by replacing the guide rods havingdifferent lengths. Moreover, each guide rod FA4 can include threadedends that can be bolted securely against the base FA1 and the distalframe FA2 and to provide a sufficient tension therebetween, as well asallowing fine height adjustments. Although the illustrated embodiment isshown with just a single pair of guide rods FA4, additional pairs ofguide rods can be included, particularly when the width of the movablesupport FA3 becomes larger, such as when the movable support FA3includes multiple lower bending members FA3 a, such as schematicallyillustrated in FIG. 6. In this respect, each of the main beams FA2 a caninclude at least a pair of through holes FA2 e spaced along the lengthdirection thereof each for receiving a guide rod FA4. See FIGS. 3 and 5.Similarly, the movable support FA3 also includes at least one pair ofthrough holes FA3 a each for receiving a guide rod. See FIGS. 1-3, 5,and 6.

Moreover, the guide rods FA4 further allow the movable support to slidetherealong as the movable support moves between the base and the upperframe. At least one lower loading member LMI is secured to the movablesupport. FIG. 1 shows a single lower loading member LMI and two upperloading members LMu to provide a three-point bending deflection system,while FIG. 6 shows two lower loading members LMI disposed at the movablesupport and two upper loading members to provide a four-point bendingdeflection system. This embodiment also can be used with just a singlelower loading member LMI (illustrated in phantom).

The movable support FA3, as well as each of the upper and lower loadingmembers LMu, LMI, can be made from a block of plastic material, such ashigh density polyethylene, aluminum, steel, wood, or a compositematerial, such as carbon fiber. Each loading member LMu, LMI can becontoured to the match the shape of the SM for better support, and has awidth that allows the SM to bend along multiple points. For example, thewidth of the loading members can range from 1 cm-15 cm, depending on thelength of the SM.

The driving mechanism DM can be any conventional commercially availablemechanical devices that can move the movable support toward or away fromthe distal frame FA2 (or away or toward the base FA1). For example, thedriving mechanism DM can be a linear jack screw, such as a worm gearscrew jack (Classic, IMA Series) available fromhttps://www.seshagirigears.com or from McMASTER-CARR(https://www.mcmaster.com/screw-jacks). Rotating a shaft, such as usinga motor or hand crank, translates a rotary motion of the crank into alinear motion, namely pushing or pulling motion.

Referring to FIGS. 1-2, a jack screw type driving mechanism can includesa main housing DM1, a driving shaft DM2, a hand crank DM3, a drivenshaft DM4, a driving-worm gear (not illustrated), a driven-worm gear(not illustrated), and a support platform DM5. The driven shaft DM4 isrotatably supported and typically includes an external acme threadingalong the substantial length thereof. The driving-worm gear is coaxiallyarranged with the driving shaft DM2, which is rotatably mounted to themain housing DM1, so that it can only rotate. That is, the driving shaftis held so that it cannot move along the axial direction of the drivingshaft. The driving-worm gear can have external worm gear teeth, whichcan be integrally formed with or mounted to the driving shaft DM2. Thehand crank DM3 is mounted to the driving shaft DM2 so that rotating thehand crank rotates the driving shaft DM2, which rotates the driving-wormgear in the direction of the rotation of the hand crank.

The driven-worm gear is coaxially arranged with the driven shaft DM4 andcan include external worm gear teeth that mesh with the external gearteeth of the driving-worm gear. The driven worm gear further includes aninternal acme threading that meshes with the external acme threading ofthe driven shaft DM4. The driven-worm gear is rotatably mounted to themain housing but is prevented from moving in the axial direction of thedriven shaft DM4. Thus, rotating the driven-worm gear via the drivingshaft DM2 (by rotating the crank DM3) rotates the driven shaft DM4. Thiscauses the driven shaft DM4 to move axially, either away from or towardthe main housing, depending on the rotation direction of the crank.Instead of the crank, a motor can be used to drive the driven shaftusing a controller or a computer.

The support platform DM5 is rotatably mounted to the distal end of thedriven shaft DM4 to enable the driven shaft DM4 to rotate relative tothe support platform DM5, such as using a bearing. The support platformDM5 supports the movable support FA3 so that raising the supportplatform DM5 raises the movable support toward the distal frame FA2.

The upper loading members LMu and the at least one lower loading memberLMI, which are configured to provide multiple bending points, aredisposed facing each other to apply an opposing force to the SM from theopposing sides. With the SM sandwiched between the upper loading membersLMu and the at least one lower loading member LMI, rotating the crankDM2 to rotate the driven shaft LM4 and elevate the support platform LM5toward the distal frame FA2, moves the movable support FA3 in the samedirection to apply a load to the SM.

The first sensor S1 can be a conventional load cell disposed between thedriving mechanism DM and the movable support FA3 for measuring the forceapplied by the driving mechanism DM to the SM. For example, the sensorS1 can be used include a load cell, such as Honeywell Model 31(https://sensing.honeywell.com/test-measurement-products/miniature-stainless-steel-load-cells/model-31)and Model 41(https://sensing.honeywell.com/test-measurement-products/stainless-steel-low-profile-load-cells/model-41).Specifically, the load cell can be disposed underneath the movablesupport FA3, between the movable support FA3 and support platform DM5 sothat it can measure the amount of force the driving mechanism DM isapplying to the SM.

The second sensors S2, on the other hand, each are mounted to the distalframe FA2 and positioned along the length thereof at desired positions,and substantially aligned centrally with the sensor S3, for measuring adeflection around the SM. Each of the sensors S2 include a contact padS2 a, where its position is adjustably displaceable longitudinally(e.g., perpendicular to the longitudinal direction of the distal frameFA2) to permit each pad to be in contact with the SM duringmeasurements. Specifically, as shown in FIGS. 1 and 3, the sensor canhave a threaded shaft and using a pair of nuts N to position the pad ata desired height. The third sensor S3, on the other hand, is anon-contact type, and uses a radio frequency antenna RFA disposed spacedfrom the SM and mounted to the distal frame FA2 to measure thedeflection of the SM without using any strain sensing device attached tothe SM and/or the material surrounding the SM. There is no contactbetween the SM and the antenna RFA.

As noted earlier, the present development uses two distinct types ofdisplacement sensors S2, S3 that can be mounted to the distal frame heldstationary in relation to the base FA1. In the illustrated embodiment,the first type can be three linear displacement sensors (hereafterdisplacement sensor), such as DVRTs (Differential Variable ReluctanceTransducer) or LVDT (Linear Variable Differential Transducer),positioned along the length of the distal frame to function as lineardisplacement sensors to measure the displacement of the material (e.g.,skin) surrounding the SM. As illustrated, two linear displacementsensors S2 are located adjacent to two upper loading members LMu whilethe third linear sensor S2 is located at the midpoint between the twoupper loading members LMu. The two outer displacement sensors LMu canmeasure the baseline displacement due to compliance in the material(e.g., soft tissue) surrounding the loaded SM. The centrally locateddisplacement sensor S2 measures the total displacement due to thematerial (e.g., soft tissue) compliance when loaded. Subtracting theaverage displacement of the two outer displacement sensors S2 from thedisplacement of the central displacement sensor S2 results in thedeflection of the loaded SM due to bending.

The displacement sensors S2, as mentioned can be DVRTs or LVDT, whichare commercially available, for example, from MicroStrain/LORD Sensing(https://www.microstrain.com/displacement/nodes) or Omega Engineering(https://www.omega.com/subsection/displacement-proximity-transducers-all.html).

Referring to FIG. 7, the second type of deflection sensor is a radiofrequency antenna RFA, which can be powered by a network analyzer 26that includes an ND converter. The network analyzer measures the S11parameter (return loss) of the antenna RFA. The resonant frequency ofthe antenna is determined as the frequency location of the local minimaof the S11 parameter dB response. The antenna's resonant frequency issensitive to the location of objects in the near field, and isespecially sensitive to metallic objects. Therefore, the antenna RFA canfunction as a proximity sensor and can measure deflections of the SM,such as extremity together with the implanted hardware, which istypically metallic.

The deflection measured from the antenna RFA is advantageous because itdoes not contact the SM and measures the deflection of the construct asa whole instead of relying on skin deflection of an extremity forexample. The displacement sensors S2 are advantageous because they canprovide displacement measurements at multiple locations along the lengthof the loaded extremity, allowing for accounting displacements due tosoft tissue compliance. The stiffness of the target construct can beobtained from the slope of the deflection curve versus the applied loadcurve.

Specifically, referring to FIGS. 3-5, the antenna RFA can have a dipoleconfiguration, using a pair of coiled coaxial cables CC1, CC2 (althoughnon-coiled type antennas can be used). But only one of the two cables,such as cable CC1, is connected to the network analyzer to obtain theS11 parameter. Using the center conductor W1, the S11 parameter, whichcan be measured with the cable CC1 connected to port 1, represents theratio of the power sourced at port 1 that is returned back to port 1,also known as the “return loss.” At the antenna's resonant frequency,the S11 parameter reaches a local minima. The outer shield of the othercable CC2 of the antenna RFA can be grounded to the outer shield of thecable CC1 that is grounded to the port 1.

For example, referring to FIG. 5, the outer shields of both coaxialcables, at one (first and third) ends thereof, can be electricallyconnected to each other at a coaxial connector CC1 a that allowsconnection of an extension coaxial cable that connects to port 1 of thenetwork analyzer. For example, this can be achieved by connecting theshield CC2 s (FIG. 5) of the cable CC2 to the connector CC1 a providedat the cable CC1. Alternatively, as illustrated in FIG. 3, the cable CC2also can include another coaxial connector CC2 a at the one (third) endand secure the two metal connectors so they make good contact and permitelectrical connection. Alternatively, connectors CC1 a can use a Y-typeconnector (not shown) to connect both the connectors CC1 a and CC2 a toground the shields to each other. Although FIG. 5 illustrates the bottomview of FIG. 3, the embodiment of FIG. 5 includes a terminating cap CC2c instead of the connector CC2 a.

It should be noted that the center conductor W2 of the cable CC2 isisolated from the coaxial connector CC1 a (or the Y-type connector). Atthe other (second and fourth) ends of the cables CC1, CC2, the outerjackets and the shields, and any foil shields are stripped off to exposethe dielectric insulators CC1 b, CC2 b. Moreover, referring to FIGS.3-5, part of each exposed dielectric insulator is stripped off to exposethe respective center conductors W1, W2.

Still referring to FIGS. 3-5, a mount M is installed on the distal frameFA2 using brackets FA2 d and fasteners. The mount M maintains theantenna RFA in a static position in relation to the distal frame FA2.Specifically, the mount M includes a pair of spaced plates M1, M2,preferably non-metallic, that are spaced apart. The pair of plates M1,M2 include a plurality of through holes, through which the cables CC1,CC2 are inserted through to provide a coiled configuration for each ofthe cables CC1, CC2.

Referring to FIG. 4, which provides an enlarged schematic view of thefree (second and fourth) ends of the cables CC1, CC2, the exposed centerconductors W1, W2 are held substantially parallel to each other. Thiscan be achieved by securing the ends of the exposed dielectricinsulators CC1 b, CC2 b in the pair of spaced through holes in the plateM2, such as by hot gluing them in place in the through holes, while theyare aligned substantially parallel. The exposed center conductors alsocan be bent to position them substantially parallel to each other. Thisallows the center conductors W1, W2 of the two coaxial cables CC1, CC2to interact with each other via their electric fields. These centerconductors W1, W2 are spaced from each other at the second and fourthends, but otherwise just terminate without connecting to anything. Onlythe center conductor W1 of the cable CC1 at the first end is used toconnect to the network analyzer 26. The length of the exposed centerconductors W1, W2 can be in the range 0.5 cm-20 cm. In the presentembodiment, the ideal length of the exposed center conductors W1, W2 isaround 4 cm.

The cable CC2 is merely a dud as, while the shields of both cables aregrounded, the center conductor of the cable CC2 is not connected to thenetwork analyzer or anything else. Although the cable CC2 is notdirectly used in signal analysis, it can help to eliminate noisecompared to a monopole antenna. For either antenna configuration, the SMaffects the frequency at which the local maxima or minima occurs whenthe SM is loaded.

Specifically, a metal plate or rod associated with the SM (e.g., afractured tibia fixed with an intramedullary nail) corresponding to SMcan be interrogated from the side of the coiled cables. Certainlocations along the length of the coaxial cable has the greatest shiftin signal. These locations depend on the resonant frequency harmonicthat is measured. The coiled shape increases the signal strength byaligning the locations along the cable length where the signal isstrongest. The antenna parameters can be optimized by adjusting theantenna length, spacing between coils, and resonant frequency harmonic,while interrogating a reference stainless steel bar at known distancesfrom the antenna. Increasing the spacing between the coils increases thesignal strength but also increases noise. Higher frequency harmonicsalso result in a stronger signal but greater noise.

Upon directing and emitting an alternating magnetic or electromagneticfield through a pre-determined frequency sweep using a source integratedin the network analyzer (or an external source if the network analyzeris not provided with the source) toward the SM, the SM interacts withthe applied electromagnetic field through near field effects. The SMbecomes electromagnetically coupled to the antenna RFA. The distancebetween the SM and the antenna can be represented by characteristics ofthe electromagnetic field coupling between the antenna and the SM. Asthe distance between the antenna and the SM changes, the characteristicsof the electromagnetic field coupling between the antenna and the SMshifts because the fundamental coupling between the antenna and the SMbecomes altered. For example, inducing deflection in the SM by applyinga load using the driving mechanism DM changes the distance between theantenna and the SM, while the antenna RFA remains fixed in spacerelative to the distal frame FA2, resulting in an alteration in theresonance frequency due to changes in the electromagnetic couplingbetween the antenna RFA and the SM.

The S-parameters of the antenna can be obtained by the connected networkanalyzer 26 or the connected analyzer/computer 28. The analyzer/computer28 can determine the resonance frequency, as well as the S-parametermagnitude of the antenna if coupled to the SM via the ND converter,without the need for any strain sensor directly attached to the SM. If aseparate source, which can be an inductor or other conventionalapparatuses for applying electromagnetic fields in the radio frequencyspectrum, the network analyzer can just be an ND converter, and theanalyzer/computer 28 can execute the functions of the network analyzer26 using software.

In operation, the antenna RFA is spaced from the SM so that it does nottouch any surface of the SM or the SM and/or surrounding material duringthe operational conditions. The electromagnetic field surrounding theantenna RFA is affected by objects in the near field range due to theconductive and/or dielectric properties of the SM. A conductive materialhas an eddy current induced, which in turn causes the material to act asan antenna itself, thus altering the electromagnetic field. Anon-conductive dielectric material can also alter the electromagneticfield via the electromagnetic polarization of the material. Therefore,the SM can be any material that is conductive and/or has a relativepermittivity (i.e., dielectric constant) that is different than therelative permittivity of the surrounding medium (e.g., air).

The components of a monitoring system 20 can include the networkanalyzer 26, such as a commercially-available Tektronix TTR503A NetworkAnalyzer and Rohde & Schwarz ZVB4, which can apply electromagneticfields in the radio frequency spectrum, the antenna interface, whichincludes the antenna RFA, including any wire extension extending fromthe antenna wires, and the analyzer 28, which can be a computer thatreads and analyzes the data output from the network analyzer or storedover a period, or otherwise receives data that has been accumulated overthe period. The operating frequency range of the present monitoringsystem can be 10 MHz to 4 GHz, with the preferred range being 40 MHz to500 MHz for biomedical applications.

FIG. 8 schematically illustrates the analyzer 28, which comprises acontroller or computer that can be programmed to analyze the shifts inthe characteristics of the electromagnetic field coupling between theantenna RFA and the SM over an evaluation period or a predeterminednumber of measurements of the characteristics of the electromagneticfield coupling between the antenna and the SM obtained over apredetermined interval. The computer includes CPU (processor) 28A,memory 2(B), I/O (input/output) interface 28C. The I/O interface 28C caninclude a communication interface, such as Ethernet, for communicationto a network and Internet, a display interface for connecting to adisplay device 21, and typical interfaces, such as USB, for connectingperipheral devices, including a keyboard and a mouse, as well as thenetwork analyzer or any other device that can obtain the frequency sweepfrom the electrical signals obtained from the antenna RFA. The networkanalyzer 26 can be either a standalone apparatus, which can also beconnected to the computer via the I/O interface 28C, or a peripheraldevice that converts the electrical signals from the antenna RFA intodigital signals (e.g., ND converter) readable by the computer, and canbe connected to the computer 28 via either the Ethernet, USB or serialport.

The computer 28 can determine the characteristics of the electromagneticfield coupling between the antenna and the SM from the electrical signaldata obtained by the network analyzer 26 across a pre-determinedfrequency range. The functions of the network analyzer are well knownand are commercially available either as a standalone unit or softwareoperated unit using an ND converter, such as a commercially availableTektronix TTR503A and Rohde & Schwarz ZVB4 network analyzers.Alternatively, the computer 28 can analyze the stored data of thecharacteristics of the electromagnetic field coupling between theantenna and the SM determined and read over an evaluation period or apredetermined number of times read over a predetermined interval by thenetwork analyzer 26. The storage device can be a memory drive within thecomputer itself, flash memory, network drive, or remote databaseconnected over the Internet.

The memory 28B communicates with the CPU 28A via a bus. The memory 28Bcan include a ROM 2861 and a RAM 12862. The memory 28B also can beconfigured as a non-volatile computer storage medium, such as a flashmemory, instead of the RAM and the ROM. The computer 28 can also includea removable memory (e.g., flash card) connected via the I/O interfaceusing, for example, USB or any other conventional memory card interface,and conventional hard disk 28D. The memory 28B and hard disk 28D aresome embodiments of a non-transitory machine-readable medium that canstore instructions, which, when executed by the processor, that performvarious operations. These operations include, but are not limited to,controlling/operating the source connected to the I/O interface 28C,controlling/operating the network analyzer connected to the I/Ointerface 28C, determining the characteristics of the electromagneticfield coupling between the antenna RFA and the SM, determining the shiftin characteristics of the electromagnetic field coupling between theantenna and the SM based on the electrical signals received from theantenna RFA in response to an electromagnetic field applied at differenttimes over the evaluation period, and determining the temporal change inthe displacement of the SM per the applied load, based on the shift incharacteristics of the electromagnetic field coupling between theantenna and the SM.

FIG. 9 illustrates an operational flow of the monitoring system that canmonitor the changes in the SM based on the antenna RFA. After the SM hasbeen secured to the loading device, at S100, the network analyzer 26 (orthe external source) is controlled to generate, emit, or direct anelectromagnetic field over a pre-defined frequency bandwidth towards theSM using the antenna RFA. The SM interacts with the emittedelectromagnetic field when subject to an alternating magnetic orelectromagnetic field. In S101, the network analyzer 26 outputs a(first) signal to the antenna RFA, which causes the antenna RFA tooutput an (second) electrical signal based on the coupledelectromagnetic field. The output electrical signal received from theantenna RFA is input to the network analyzer 26. After either thenetworks analyzer 26 or the controller determines the initialcharacteristics of the electromagnetic field coupling between theantenna RFA and the SM, which represents these parameters at the initialor start period, they can be stored in any of analyzer/computer 28,remote database, or local/portable storage device.

At S102, after determining the initial characteristics of theelectromagnetic field coupling between the antenna RFA and the SM, atimer or a counter is reset (i.e., evaluation starting point at whichthe initial characteristics of the electromagnetic field couplingbetween the antenna RFA and the SM has been set as a reference point).For accurate positioning of the SM after setting the reference, theinitially read (reference) parameters can be used as a reference toaccurately position the SM for taking subsequent multiple measurements.The timer/counter can be set using the controller or a standalone timer.Alternatively, the technician monitoring the target area can keep acalendar or manually keep track of the time and date as to when thecharacteristics of the electromagnetic field coupling between theantenna RFA and the SM are read in relation to the applied load. At apredesigned or desired time interval after the initial characteristicsof the electromagnetic field coupling between the antenna RFA and the SMhave been determined, process/step S103 essentially repeats S100, andprocess/step S104 (corresponding to S204 in FIG. 10) repeatsprocess/step S101 to determine the current characteristics of theelectromagnetic field coupling between the antenna RFA and the SM underload applied by the driving mechanism DM.

At S105, the analyzer or computer 28 can analyze the previouslydetermined characteristics of the electromagnetic field coupling betweenthe antenna RFA and the SM and the currently determined characteristicsof the electromagnetic field coupling between the antenna RFA and the SMand determine the shift of the characteristics of the electromagneticfield coupling between the antenna RFA and the SM by comparing thedetermined characteristics of the electromagnetic field coupling betweenthe antenna RFA and the SM over different times. The shift correspondsto the change in the distance between the antenna RFA and SM during theloading. Since the data is stored, the shift in the characteristics ofthe electromagnetic field coupling between the antenna RFA and the SMcan be determined after a predetermined number of characteristics of theelectromagnetic field coupling between the antenna RFA and the SM over adesired evaluation period has been read, or after the desired evaluationperiod has lapsed (where a desired total number of characteristics ofthe electromagnetic field coupling between the antenna RFA and the SMfor the desired evaluation period has been determined) (see S107). Inthis respect, the slope can be calculated from the change in thedistance versus the applied loads (curve).

At S106 the analyzer 28 determines whether the evaluation period haslapsed or the desired total number of characteristics of theelectromagnetic field coupling between the antenna RFA and the SM hasbeen made after determining each of the characteristics of theelectromagnetic field coupling between the antenna and the SM other thanthe determination of the initial characteristics of the electromagneticfield coupling between the antenna RFA and the SM. If the negative (NOin S106), after the preset interval, which corresponds to the durationor the interval between evaluations over the evaluation period, haslapsed (YES) at S108, processes/steps S103-S105 are repeated untilaffirmative in process/step S106 (YES). If affirmative (YES in S106),the analyzer 28 ends the evaluation since the evaluation period haslapsed. The temporal changes in relative displacement of the SM aredetermined based on the determined shift over the evaluation period. Theactual temporal changes in the displacement of the SM can be determinedby implementing an a priori deformation-electrical parameter ordisplacement-electrical parameter calibration of the hardware. Data froman electrical parameter-deformation or electrical parameter-displacementcalibration, performed in advance, can be stored in memory 28Baccessible by the analyzer 28.

For example, the electrical parameter signal, such as resonantfrequency, can be calibrated to correspond to the distance of theantenna RFA from surface of the SM. This can be done by measuring theactual distance from the SM in relation to the resonant frequency. Theresonant frequency measurement can then be used to determine therelationship between distance and resonant frequency. Because therelationship is known between the resonant frequency and the distance,the relationship can be determined between the resonant frequency andthe distance. The resonant frequency measurement can therefore becalibrated to give a direct measure of the distance for that particularSM, environment, and antenna setup, namely making the antenna functionas a displacement sensor.

The shift in characteristics of the electromagnetic field couplingbetween the antenna RFA and the SM can be used rather than the absolutevalues of the determined characteristics of the electromagnetic fieldcoupling between the antenna RFA and the SM in determining the temporalrelative changes in displacement of the SM. Based on the temporalchanges in relative displacement of the SM, changes in the target areaor biological subject can be determined. For instance, for a fracturefixation plate implanted in a person, these changes can be monitored foruse in the diagnosis and the prognosis for the healing of a fracturedbone for instance, or a condition of the SM.

When applying the present methodology to fracture healing, the objectiveis to determine the level of healing that has occurred by testing themechanical stability of the bone-implant construct. As the healingprogresses, the stiffness increases. As the stiffness increases, thedeflection of the construct relative to the applied load decreases, andthe signal from the antenna, such as resonant frequency shift, alsodecreases because it is a measure of the construct deflection.Calculating the shift in resonant frequency relative to the load appliedto the bone-implant construct provides a measure of the constructstiffness and thus the shift in the mechanical stability. Therefore, theslope of the resonant frequency versus the applied load curve can becalculated. This slope is a good indicator of the stiffness. Bydetermining this slope over time and comparing it to initialmeasurements, one can determine how the fracture is healing over thattime frame.

This methodology is particularly useful for monitoring the relative loadon the implant (due to the stiffness of the bone) at predetermined timepoints, such as every two weeks, throughout the healing of the fractureto monitor or predict the healing progress. As a fracture heals, the newtissue that grows progressively stabilizes the fracture, and thereforeincreases the relative load borne by the bone and decreases the relativeload borne by the implant (orthopedic plate or intramedullary nail). Asthe load on the plate decreases, the deformation of the implantdecreases proportionally, and the characteristics change accordingly,reflecting the stiffness of the bone. Calculating the shift in thesignal, such as resonant frequency, relative to the load applied to theimplant-bone construct reflects a measure of the relative load on theimplant, which corresponds to the amount of deflection or change in theamount of flexing. Therefore, the signal from the antenna can be plottedagainst the load applied to SM and the change in the deflection amountor the amount of distance moved by the load. The slope of the resultingcurve can represent the stability of the SM and the level of healing. Ifthe fracture is not healing properly, the load on the plate changesslowly or does not change over time, which means the amount ofdeflection does not change. By taking temporal measurements, a physiciancan monitor the healing progress by determining the change in thedeflection amount relative to the initial measurement. The measurementcan therefore provide the physician with an early indicator whether thefracture is not healing normally and may need further treatment.

While the data from the antenna RFA is being collected, the data fromthe load sensor S1 and the displacement sensors S2 can be collectedconcurrently or serially from a signal conditioner(s) 24, which isconnected to the ND converter (or network analyzer) 26 that converts theanalog signals output by these sensors to digital signals via ports 3-6.Some examples of signal conditioners include, for the DVRT displacementsensors, LORD MicroStrain, model DEMOD-DC(https://www.microstrain.com/displacement/DEMOD-DC) or model DEMOD-DVRT(https://www.microstrain.com/displacement/DEMOD-DVRT-2).

The load force data output by the load sensor S1 and the deflectionsamount output by the displacement sensors S3 all can be tracked, whilecalculating the shift in signal obtained via the antenna RFA via theport 1 of the network analyzer 26. Specifically, referring to FIG. 10,at process/step S202, after the reset and the timer has been initiatedin S102 in FIG. 9, the driving mechanism DM applies a desired load tostart the evaluation. In process/step 204, which corresponds to S104timewise, the network analyzer 26 receives the electrical signals fromthe sensors S1, S2, and S3. In process/step S206, which corresponds toS104 timewise, the network analyzer 26 receives the applied loadcorresponding to the signal from the load sensor S1 and the displacementcorresponding to the signal from each of the displacement sensors S2. Inprocess/step S208, the analyzer 28 calculates the slope from thedisplacement data (stored) from each of the displacement sensors S2versus an applied load curve (multiple applied loads). In process/stepS210, which corresponds to S106, the analyzer 28 determines whether theevaluation period or the desired total number of characteristics of theelectromagnetic field coupling between the antenna RFA and the SM haslapsed. Process/step S212 corresponds to S108. If the evaluation periodhas not lapsed, S202-S210 are repeated until affirmative YES in S210(i.e., the evaluation period ends). If affirmative in S210, the analyzer28 ends the evaluation.

In the present development, testing was conducted on a bone, namely afractured tibia fixed with an intramedullary nail or rod, which ismetal. But it should be noted that the SM to be tested is not limited tobone medium, but can be applied to any structural material.

Testing I: Healthy Human Tibia

The loading device was tested on a healthy tibia of a volunteer. Theloading frame was set in a three-point bending configuration asillustrated in FIG. 1 with the upper two loading members LMu 35 cmapart, and the lower loading member LMI at the middle thereof. Theloading was applied using the driving mechanism DM for five cycles from0 to 100 N. Separate tests were conducted using the antenna RFA and thedisplacement sensors S2. The antenna test showed a consistent linearresponse of the resonant frequency relative to the applied load, asshown in FIG. 11. The mean slope from five cycles of loading was −0.082MHz/N, and the standard deviation was 0.0027 MHz/N, which is 3% of themean.

The displacement sensors S2 were tested in a series of experiments toevaluate the corrected displacement measurement used to account forcompliance of the soft tissue. The corrected displacement was defined asthe center displacement minus the average of the two outsidedisplacements. Experiments were conducted using three methods to achievevarious degrees of compliance. First, the loading device was applied inits normal state (A). Second, the loading was applied in a padding state(B) where foam padding was added to the loading points to increase thecompliance. Third, the loading device was applied in a strap state (C)where the padding was removed, and the loading points were strapped tothe tested leg and tightened to decrease the compliance. The leg wasloaded for five cycles, and each test was conducted for threerepetitions. Slopes of the displacement/load curves were calculated forboth the raw center displacement sensor's displacement and the correcteddisplacement.

Referring to FIG. 12, as expected, the center displacement was highlyvariable between the three compliance groups (A)-(C) because the centerdisplacement includes the displacement due to compliance and thedeflection due to bending. The displacement due to compliance should bevariable between the three groups, but the bending deflection wastheoretically the same. Because the corrected displacement should be ameasure of only the bending deflection, it should be the same betweenthe three groups. The results showed that the corrected displacement wasless than the center displacement and had lower standard deviations thanthe center displacement. There were also smaller differences between thethree groups for the corrected displacement, compared to the centerdisplacement. The range of the three groups was 30% of the mean, and 83%of the mean for the corrected and center displacements, respectively.The corrected displacement therefore is an improvement over the rawcenter displacement for lowering the variability and accounting forchanges in compliance.

Additional experiments used the corrected displacement sensorsdisplacement method to test three loading configurations, where the twoupper loading members were spaced 35 cm, 23 cm, and 18 cm apart, whileusing a single lower loading member located centrally of the two upperloading members. Changing the spacing changes the applied bending momentrelative to the applied load, while changing the deflection due tobending. Experimental results were compared to analytical solutions ofbeam defection (δ) due to three-point bending, which were calculatedusing the equation:

${\delta = \frac{Pl^{3}}{48{EI}}},$

where P is the applied load, l is the length between the upper loadingpoints, E=20 GPa is the elastic modulus of cortical bone according toYoung's Modulus of Trabecular and Cortical Bone Material: Ultrasonic andMicrotensile Measurements, Jae Young Rho, Richard B. Ashman, and CharlesH. Turner, 1993 Journal of Biomechanics, Volume 26, Issue 2: pp.111-119, and I=1.5 E⁻⁸ m⁴ is the area moment of inertia of a human tibiaaccording to The Human Tibia A Simplified Method of RadiographicAnalysis of its Cross-Section, with Anthropometric Correlations, Ira D.Stein and Gerald Granik, 1979 Annals of Biomedical Engineering, Volume7, pp. 103-116.

Analytical solutions were also calculated for titanium rod of 10 mm and12 mm diameters representing an unhealed (post-operational) fracturefixed with an intramedullary rod to compare the testing results on anintact tibia to the expected results for a fractured tibia. Theexperimental results showed relatively good agreement with theanalytical solutions in FIG. 13. Discrepancies were likely due to themoment of inertia and elastic modulus properties assumed in theanalytical solution, which can be highly variable. Large differenceswere seen between the analytical solutions for the intact tibia and thefractured tibia (intramedullary rod), indicating that differences in thebone deflection can be tracked over time as a fracture heals.

Testing II: Fracture Model of Sheep Tibia

The loading device was also tested on a cadaveric sheep tibia with animplanted intramedullary rod. The tibia was tested in eight conditionsto simulate various stages of fracture healing. The first test wasconducted with rod implanted in an intact bone. For tests two throughsix, progressively larger osteotomies were made to simulate a partiallyhealed transverse diaphyseal fracture. For test seven, the osteotomy wasextended entirely through the cross section of the bone to simulate anunhealed (post-operational) fracture. For test eight, a second completeosteotomy was made to simulate an unhealed segmental fracture.Radiographs were used to guide and confirm the osteotomies. See FIG. 14.

The loading device was configured in three-point bending deflection withthe two upper loading members spaced 18 cm apart and a single lowerloading member located centrally of the two upper loading members.Deflection data were collected from the antenna RFA and the displacementsensors S2 simultaneously. In each test, the bone was loaded from 30 Nto 130 N for five cycles, and slopes were calculated from the correcteddisplacement sensor displacement versus load curves and the antennaresonant frequency versus the same load curves.

Referring to FIGS. 15A-15B, results using the antenna data showed thatthe resonant frequency shift increased relative to the applied load witheach progressive osteotomy condition, indicating that the slope of theresonant frequency versus the load curve was able to track the decreasedstiffness of the bone-rod construct as it was progressivelydestabilized. The displacement sensor data showed a similar trend to theantenna data, where the slope of the displacement versus the same loadcurve was higher in tests six through eight than tests one through five.But the slopes did not increase in each of the first five tests testlike they did for the antenna data. Also, tests eight (segmentalfracture) had a lower displacement/load slope than test seven (singlefracture).

The greater variability in the displacement sensor data than the antennadata can be attributed to the displacement sensor measuring skindisplacement, whereas the antenna RFA measures the combined deflectionof the bone, soft tissue, and implanted metal rod, and is especiallysensitive to metal materials. The large amount of soft tissue around thesheep tibia relative to typical human tibias may have contributed to thevariability in the displacement sensor data. Also, the displacement/loadslope was likely lower for test eight than test seven because the centerdisplacement sensor S2 was located over the loose segment of thesegmental fracture and did not displace as much, despite the moredestabilized structure. But the antenna data did track the increaseddestabilization of test eight as an increased slope in the resonantfrequency versus the load curve. Overall, these results indicate thatthe present device is capable of tracking changes in construct stiffnessfor a fractured bone fixed with orthopedic hardware, especially usingthe antenna deflection measurement method.

In short, the above tests showed that the using both sensor types canmonitor fracture healing progress over time.

Each antenna disclosed in the previously mentioned U.S. Pat. Nos.10,674,954 and 10,641,664 is directed to monitoring for deflection at asingle region. Multiple antennae, including antenna designs described inthese patents, however, can also be configured and implemented to detectthe SM deflection at multiple locations using a single network analyzeror spectrum analyzer by connecting multiple antennae to different portsthereof and communicating with the antennae in a specified ordersequence. See FIGS. 17C-17E. Although a single network analyzer can beused to measure deflection/displacement at a single region at a time, ameasurement from each antenna would not occur concurrently. Moreover,the frequency at which measurements can be obtained from all antennaecould limit this method's applicability in instances where largetemporal resolution is necessary.

FIG. 16 schematically illustrates an embodiment of an antenna S3 a thatcan concurrently detect deflections of the SM in multiple discreteregions of the SM by configuring different regions to output uniquesignature signals at different harmonic numbers. FIG. 20 illustrates aspecific embodiment of an antenna S3 b that is configured to detect theSM deflections at two regions, similar to the embodiment of FIG. 16. InFIG. 20, the SM is disposed confronting closely to the antenna S3 b forillustration purposes only.

FIGS. 17A-17B illustrate another antenna S3 c that is configured todetect the SM deflections at three regions. In these embodiments, byconfiguring the first-third coil regions S3 c 1, S3 c 2, S3 c 3confronting the SM (first-third predetermined regions) uniquely toprovide unique signature signals at those regions, a single antenna canbe used to concurrently detect deflections at different regions of theSM. Configuring a single antenna to have unique signal signature regionsthat can be distinguished among the multiple regions being concurrentlymonitored is highly beneficial since only a single network analyzerwould be needed, as opposed to requiring use of multiple networkanalyzers or spectrum analyzers and/or the use of multiple ports on anetwork analyzer or spectrum analyzer.

FIG. 17C illustrates an antenna configuration that uses a plurality ofantennae connectable to a single network analyzer with the correspondingnumber of ports (one for each antenna) or a plurality of networkanalyzers, in a state where no load is applied to the SM. FIGS. 17D-17Eillustrate a state where the antennae configuration of FIG. 17C isapplied with a load. That is, the embodiments of FIGS. 17D-17E use thesame antenna configuration of FIG. 17C. But in the embodiment of FIG.17E, each of the three antennae is connected to its own network analyzer(which merely needs a single port) 26, while the embodiment of FIG. 17Duses a single network analyzer 26 with at least three ports eachconnected one of the three antennae.

Performing diagnostic antenna measurements concurrently at multiplespatial locations is beneficial. While the embodiment of FIGS. 17A-17Bcan measure deflections at multiple locations via a single antenna,FIGS. 17C-17E illustrate another antenna configurations that canconcurrently detect SM deflections at multiple regions using multiplediscrete antennae each of which can be configured according to theembodiments of the antennae S3, S3 a, or S3 b. In particular, FIGS.17C-17E illustrate three antennae that can be independently configuredto provide unique signature signals associated with respective threeregion where each antenna is located. Instead of the first-third coilregions S3 c 1, S3 c 2, S3 c 3 confronting the SM (first-thirdpredetermined regions) uniquely to provide unique signature signals atthose regions in, in the embodiments of FIGS. 17C-17E, three discreteantennae can replace the first-third coil regions S3 c 1, S3 c 2, S3 c3, to be used to concurrently detect deflections at the threecorresponding regions of the SM. Accordingly, each of the three antennaecan have unique signal signature regions that can be distinguished amongthe multiple regions being concurrently monitored.

The measurement can also be achieved using a combination of theaforementioned techniques in these embodiments. Moreover, althoughmeasurements at each antenna location can be performed concurrently,they can be sequentially measured one after another. The number ofmeasurement locations is not limited, and can be increased (ordecreased) by adding additional (or subtracting) antennae.

One beneficial application occurs in orthopaedics where a fixation plateis used to stabilize fractured bone. The fixation plate and fracturedbone act as a composite SM. The antenna S3 c can be disposed confrontingthe SM at a fixed reference distance using a support structure, whichcan be an antenna housing or frame Ma in proximity to the SM. Here, thefirst coil region S3 c 1 is disposed at a distance δ_(A1), the secondcoil region S3 c 2 at δ_(B1), and the third coil region S3 c 3 at δ_(C1)at rest (when no load is applied to the SM). These distances δ_(A1),δ_(B1), δ_(C1) all can be set the same or differently to produce uniquesignature signals at each respective region.

The SM can be loaded in tension, compression, or bending using, forexample, by a mechanical force, patient body weight, muscularcontraction, pneumatic actuation, or hydraulic force. Those skilled inthe art would understand that various loading can be used to producetension, compression, or bending in the SM. The loading on thebone-fixation plate SM causes the SM to displace relative to theantenna, which is maintained in a fixed position relative to the SM atrest. The load on SM causes the SM to deflect, namely moving the initialdistances δ_(A1), δ_(B1), δ_(C1) to δ_(A2), δ_(B2), δ_(C2) atpredetermined points of the SM. The deflection is detected as a shift inthe antenna's resonant frequency. Repeating this measurement throughoutthe collection period corresponding to the post-operative healing periodenables quantification of temporal changes to the stiffness of the SM,and thus provides insight to the quality of bone healing.

FIGS. 18A-18C reveal data from the antenna S3 c of FIGS. 17A-17B that amovement of the SM at some locations along the antenna can cause largechanges to the resonant frequency at certain locations, whilesimultaneously eliciting little to no change when measuring at adifferent resonant frequency. An antenna that uses two parallel coaxialcables can detect deflection of an adjacent structure at multiplepoints. Specifically, selective antenna coiling configuration can alignregions of maximum and minimum sensitivity to deflection at therespective regions to allow monitoring at multiple discrete regions.

The present monitoring system can determine the relative displacement ofthe SM, which can include beams, rods, or orthopaedic hardware. A sourceconnected to the antenna, such as a network analyzer 26, generates anelectromagnetic field emitted by the antenna. The electromagnetic fieldin a predetermined radio frequency spectrum, such as 10 Mhz to 4 GHz,with the preferred range being 40 Mhz to 900 MHz for biomedicalapplications, is emitted based on the antenna design. The antennareceives signals from the electromagnetic field, and thus can detectchanges to the pattern of the electromagnetic field resulting fromchanges in the distance between the antenna and the SM and/or adeflection of the SM. Thus, shifts in the coupling between the SM andantenna during mechanical loading of the SM are indicative of deflectionor deformation of the SM.

Analysis of the electrical signal output from the antenna via one of thepair of the coaxial cables characterize the electromagnetic couplingbetween the antenna and the SM. Such electrical signals include, but arenot limited to, resonant frequencies, impedance, and the responsemagnitude of the S parameter. The electrical signal data are collectedover a collection period so that temporal changes to the antenna-SMcoupling may be quantified. Thus, repeated measurements during thecollection period quantify temporal changes in the SM deflection due toa given mechanical loading.

Stiffness represents the resistance of the SM to deflection resultingfrom a given force. For example, increasing the stiffness of the SMdecreases its deflection for any given load. Therefore, use of themonitoring system to quantify the SM deflection during mechanicalloading allows for a relative quantification of the SM's stiffness.Performing this measurement over a collection period allows formeasurement of temporal changes to the SM's stiffness.

As previously mentioned, the antenna can be a dipole antenna where twopoles of the antenna are utilized. The pair of poles can be parallel toeach other. Each pole may be a coaxial cable. The grounds of each cableare connected together and one of the coaxial cables is connected to anetwork analyzer port. The network analyzer provides power and measuresthe returned power at that port. The ratio of these two values is the“return loss”, also known as the S11 parameter. The antenna's resonantfrequencies occur where the S11 parameter reaches local minima.

The resonant frequency changes based upon the distance between theantenna and SM. For this dipole antenna, there are multiple resonantfrequencies that occur harmonically based upon the coaxial cable lengthaccording to the following equation:

${{f(H)} = \frac{H*c}{2*L_{\max}}},$

where f(H) is the resonant frequency for harmonic H, c is the speed oflight in a vacuum, and L_(max) is the total length of the coaxial cable.This approximate relationship can be used to select the coaxial cablelength and harmonics to produce measurable resonant frequencies inspecified frequency ranges, such as those set by the Medical DeviceRadiocommunication Service.

The antenna cables are coiled and the SM can be interrogated from theside of the coiled cables. Certain locations along the length of thecoaxial cable have the greatest shift in the resonant frequency for agiven change in antenna-SM separation. Thus, these locations feature thegreatest sensitivity to the SM deflection. The quantity of theselocations is equal to the resonant frequency harmonic number beingmeasured. These locations of greatest sensitivity can be calculatedusing the equation:

${{{L_{s{ensitive}}(i)}\overset{{{{for}\mspace{14mu} i} = {1\mspace{14mu}\ldots\mspace{14mu} H}}\mspace{14mu}}{\Longrightarrow}\frac{2\left( {i - 1} \right)}{{2H} - 1}}*L_{\max}},$

where i is the maximum sensitivity point number, L_(sensitive)(i) is thedistance of the maximum sensitivity point from the unconnected end ofthe coaxial cable, H is the resonant frequency harmonic number, andL_(max) again is the total length of the coaxial cable. The coiled shapeof the antenna increases the signal strength by aligning multiple ofthese locations of maximum sensitivity. Although the signal strength canbe further increased by increasing the spacing between coils ormeasuring at higher harmonics, they also increase data noise levels.

Referring to FIG. 20, which illustrates the antenna S3 b that detects aSM deflection at two locations, the relationship between the locationalong the antenna and the resultant sensitivity to SM deflection is theharmonics. Specifically, a first (upper) region SMR1 of the SM and asecond (lower) region SMR2 of the SM are spaced vertically from eachother. The illustrated distance scale 300-450 is in mm. A Clamp holdsthe SM at a desired fixed distance from the antenna (schematically shownspaced at about 2-3 mm from the first coil region S3 b 1 and S3 b 2,respectively, according to the scale).

There are equal numbers of locations with the maximum sensitivity orminimum sensitivity for a given resonant frequency harmonic number(1^(st), 2^(nd), 3^(rd), 4^(th), 5^(th), etc.). The antenna thus can beuniquely configured at each region of the multiple regions to bemonitored, namely coiling differently at the multiple discrete regionsfrom each other to form each region that is highly sensitive at someharmonic numbers, while simultaneously being minimally sensitive withinthe same region at other harmonic numbers. Such arrangement of theantenna allows for a single antenna to discretely measure SM deflectionat multiple spatial locations by simultaneously interrogating frequencybandwidths featuring multiple resonant frequency harmonics with a singlenetwork analyzer port.

FIG. 21 Illustrates the radio frequency spectrum for the antenna S3 billustrated in FIG. 20 that is configured to perform simultaneousmeasurements at two locations. The resonant radio frequencies of eachharmonic shift independently when a SM is displaced in their individualvicinities. The measurement was taken from one (upper) location S3 b 1located 20% along the total length of the cable from the free end (seeFIG. 4), for the third harmonic, and another (lower) location S3 b 2located at 34% along the total length of the antenna from the free end,for the fifth harmonic. This figure shows that the two locations eachcan have a unique signal signature at different frequency harmonics.

FIGS. 22A-22B Illustrate the quantity of resonant radio frequency shiftdue to the displacement of a stainless steel SM from contacting adiscrete location of the antenna (i.e., the lower or upper location S3 b1, S3 b 2) to 10 mm displacement. Measurements were obtained using theantenna illustrated in FIG. 20, which is configured to measure at twolocations using the third and fifth harmonic frequencies. The SM wasmoved relative to the antenna locations S3 b 1, S3 b 2 and held usingthe clamp. Deflection of the SM at one antenna location (S3 b 1 or S3 b2) results in minimal shift to the resonant radio frequency at theharmonic associated with the other antenna location (S3 b 2 or S3 b 1).

Although the present development has been described in terms ofdeflecting the SM relative to the antenna, the antenna can be driven inrelation to the SM instead, such as using lead screw, pulley system,pneumatic actuator, hydraulic actuator, etc. Implementing a mechanism todisplace the antenna relative to the SM allows a single antenna andnetwork analyzer or spectrum analyzer to measure thedeflection/displacement of the SM at multiple locations simultaneously.

Given the present disclosure, one versed in the art would appreciatethat there may be other embodiments and modifications within the scopeand spirit of the present invention. Accordingly, all modificationsattainable by one versed in the art from the present disclosure withinthe scope and spirit of the present invention are to be included asfurther embodiments of the present invention. The scope of the presentinvention accordingly is to be defined as set forth in the appendedclaims.

What is claimed is:
 1. A method of monitoring changes in a structuralmember (SM) at multiple locations as the SM undergoes a displacement asa load is applied to the SM, the method comprising: an antennaeproviding step of providing a plurality of discrete antennae each with aplurality of coils, each of the plurality antennae including: a firstantenna with a first coil region configured to provide a maximumsensitivity to a first resonant frequency harmonic number within apredetermined radio frequency spectrum; and a second antenna with asecond coil region configured to provide a maximum sensitivity to asecond resonant frequency harmonic number, which is different from thefirst resonant frequency harmonic number, within the predetermined radiofrequency spectrum; an antennae disposing step of disposing the firstcoil region confronting a first predetermined region of the SM to bemonitored at a first distance from the SM and the second coil regiondisposed confronting a second predetermined region of the SM to bemonitored, which is different from the first predetermined region, at asecond distance from the SM; an inducing step of inducing, by applying afirst electrical signal to the plurality of antennae, a magnetic orelectromagnetic field in the SM and create a coupling of the magnetic orelectromagnetic field between each of the plurality of antennae and theSM, wherein characteristics of the magnetic or electromagnetic fieldcoupling between: the first antenna and the SM are associated with adistance between the first predetermined region of the SM and the firstcoil region; and the second antenna and the SM are associated with adistance between the second predetermined region of the SM and thesecond coil region; and an outputting step of outputting, by each of theplurality of antennae, a second electrical signal representing thecharacteristics of the magnetic or electromagnetic field couplingbetween the respective antenna and the SM, wherein the second electricalsignals from the first and second antennae contain information fordetermining respective deflections at the first and second predeterminedregions of the SM.
 2. The method according to claim 1, furthercomprising a first determining step of determining, based on theobtained second electrical signals from the first and second antennae,determining the respective deflections at the first and secondpredetermined regions of the SM.
 3. The method according to claim 2,further comprising a repeating step of repeating the inducing step, theoutputting step, and the first determining step at a predeterminedinterval for an evaluation period.
 4. The method according to claim 3,further comprising a second determining step of determining a shift incharacteristics of the magnetic or electromagnetic field couplingbetween each of the first and second antennae and the SM over theevaluation period.
 5. The method according to claim 4, furthercomprising a third determining step of determining a temporal change inrelative displacement of the SM at the first and second predeterminedregions of the SM over the evaluation period based on the shiftdetermined in the second determining step.
 6. The method according toclaim 5, wherein the temporal change in relative displacement of the SMat the first and second predetermined regions over the evaluation periodis represented as a slope of resonant frequency/load at the respectivefirst and second resonant frequency harmonic numbers within thepredetermined radio frequency spectrum.
 7. The method according to claim1, wherein: the SM is mountable to a target object to be monitored, theSM undergoing the displacement as the target structural object undergoesa displacement or deformation, and the first and second distances areequal to each other or different from each other.
 8. The methodaccording to claim 2, further comprising: a loading step of applying apredetermined load to the SM, wherein the first determining stepanalyzes the determined characteristics in relation to the predeterminedload applied to the SM in the loading step to determine the deflectionsat the first and second predetermined regions of the SM.
 9. The methodaccording to claim 1, wherein the outputting step outputs the secondelectrical signal from each of the plurality of antennae to at least onenetwork analyzer provided with a plurality of ports.
 10. The methodaccording to claim 1, wherein the outputting step outputs the secondelectrical signal from each of the plurality of antennae to each of aplurality of network analyzers.
 11. An apparatus for measuringdeflections in a structural member (SM) at multiple locations as the SMundergoes a displacement as a load is applied to the SM, the apparatuscomprising: a plurality of discrete antennae each with a plurality ofcoils, each of the plurality of antennae including: a first antenna witha first coil region configured to provide a maximum sensitivity to afirst resonant frequency harmonic number within a predetermined radiofrequency spectrum; and a second antenna with a second coil regionconfigured to provide a maximum sensitivity to a second resonantfrequency harmonic number, which is different from the first resonantfrequency harmonic number, within the predetermined radio frequencyspectrum; and a support structure that maintains the first coil regiondisposed confronting a first predetermined region of the SM to bemeasured at a first distance from the SM and the second coil regiondisposed confronting a second predetermined region of the SM to bemeasured, which is different from the first predetermined region, at asecond distance from the SM, wherein each of the plurality of antennaeis configured to: induce, upon applying a first electrical signalthereto, a magnetic or electromagnetic field in the SM and create acoupling of the magnetic or electromagnetic field between the respectiveantenna and the SM, wherein characteristics of the magnetic orelectromagnetic field coupling between: the first antenna and the SM areassociated with a distance between the first predetermined region of theSM and the first coil region; and the second antenna and the SM areassociated with a distance between the second predetermined region ofthe SM and the second coil region; and output a second electrical signalrepresenting the characteristics of the magnetic or electromagneticfield coupling between the respective antenna and the SM, wherein thesecond electrical signals from the first and second antenna containsinformation for determining respective deflections at the first andsecond predetermined regions of the SM.
 12. The apparatus according toclaim 1, wherein: each of the plurality of antennae is a dipole antennathat operates as a displacement sensor and comprises a first coaxialcable including a first end and a second end, the first end of the firstcoaxial cable receives the first electrical signal, and the first end ofthe first coaxial cable outputs the second electrical signal.
 13. Theapparatus according to claim 12, wherein: the dipole antenna furtherincludes a second coaxial cable including a third end and a fourth end,the first coaxial cable includes a first shield and a first centerconductor, the second coaxial cable includes a second shield and asecond center conductor, the first and second shields are electricallyconnected, the first center conductor at the second end and the secondcenter conductor at the third and fourth ends terminate without anyconnection, a predetermined length of the first center conductor isexposed at the second end and a predetermined length of the secondcenter conductor is exposed at the fourth end, and the support structuremaintains the exposed first and second conductors substantially parallelto each other.
 14. The apparatus according to claim 12, wherein: theplurality of coils include a plurality of first coils of the firstcoaxial cable and a plurality of second coils of the second coaxialcable, and the support structure maintains the plurality of first coilsin a coiled configuration between the first end and the second end andthe plurality of second coils in a coiled configuration between thethird end and the fourth end.
 15. A monitoring system for monitoringchanges in a structural member (SM) at multiple locations as the SMundergoes a displacement as a load is applied to the SM, the systemcomprising: a plurality of discrete antennae each with a plurality ofcoils, each of the plurality of antennae including: a first antenna witha first coil region configured to provide a maximum sensitivity to afirst resonant frequency harmonic number within a predetermined radiofrequency spectrum; and a second antenna with a second coil regionspaced from the first coil region and configured to provide a maximumsensitivity to a second resonant frequency harmonic number, which isdifferent from the first resonant frequency harmonic number, within thepredetermined radio frequency spectrum; and a support structure thatmaintains the first coil region disposed confronting a firstpredetermined region of the SM to be monitored at a first distance fromthe SM and the second coil region disposed confronting a secondpredetermined region of the SM to be monitored, which is different fromthe first predetermined region, at a second distance from the SM; and aload applying mechanism configured to apply a load to the SM, whereineach of the plurality of antennae is configured to: induce, uponapplying a first electrical signal thereto, a magnetic orelectromagnetic field in the SM and create a coupling of the magnetic orelectromagnetic field between the respective antenna and the SM, whereincharacteristics of the magnetic or electromagnetic field couplingbetween: the first antenna and the SM are associated with a distancebetween the first predetermined region of the SM and the first coilregion; and the second antenna and the SM are associated with a distancebetween the second predetermined region of the SM and the second coilregion; and output a second electrical signal representing thecharacteristics of the magnetic or electromagnetic field couplingbetween the respective antenna and the SM, each time the load applyingmechanism applies the load to the SM, wherein the second electricalsignals from the first and second antennae contains information fordetermining respective deflections at the first and second predeterminedregions of the SM.
 16. The monitoring system according to claim 15,further comprising: a controller including a memory storing instructionsand a processor configured to implement instructions stored in thememory and execute a plurality of tasks; at least one hardware interfaceconfigured to receive the second electrical signal from each of theplurality of antennae, and convert each second signal that is readableby the controller, wherein the plurality of tasks include: a firstdetermining task that, based on the converted second electrical signalsof the first and second antennae from the at least one hardwareinterface, determines the characteristics of the magnetic orelectromagnetic field coupling between the first and second antennae andthe SM respectively at the first and second predetermined regions of theSM, and determines the deflections at the first and second predeterminedregions of the SM, respectively; and a storing task that stores thedetermined deflections in the storage device each time the load applyingmechanism applies a load to the SM.
 17. The monitoring system accordingto claim 16, wherein the at least one hardware interface is a networkanalyzer provided with a plurality of ports or a plurality of networkanalyzers, each configured to: output the first electrical signal toeach of the plurality of antennae; receive the second electrical signalfrom each of the plurality of antennae; and also determine thecharacteristics of the magnetic or electromagnetic field couplingbetween each of the plurality of antennae and the SM based on thereceived respective second electrical signal.
 18. The monitoring systemaccording to claim 16, wherein: the first determining task determinesthe deflections at the first and second predetermined regions of the SMat a predetermined interval for an evaluation period, the storing taskstores the determined deflections at the predetermined interval for theevaluation period.
 19. The monitoring system according to claim 18,wherein the plurality of tasks include a second determining task thatdetermines a shift in characteristics of the magnetic or electromagneticfield coupling between each of the plurality of antennae and the SM overthe evaluation period.
 20. The monitoring system according to claim 19,wherein the plurality of tasks include a third determining task thatdetermines a temporal change in relative displacement of the SM at eachof the first and second predetermined regions of the SM over theevaluation period based on the shift determined in the seconddetermining task.
 21. The monitoring system according to claim 20,wherein the temporal change in relative displacement of the SM at thefirst and second predetermined regions over the evaluation period isrepresented as a slope of resonant frequency/load at the respectivefirst and second resonant frequency harmonic numbers within thepredetermined radio frequency spectrum.
 22. The monitoring systemaccording to claim 15, wherein: the load applying mechanism applies apredetermined load to the SM, the first determining task analyzes thedetermined characteristics in relation to the predetermined load appliedto the SM by the load applying mechanism to determine the deflections ateach of the first and second predetermined regions of the SM.
 23. Themonitoring system according to claim 16, wherein: each of the pluralityof antennae is a dipole antenna that operates as a displacement sensorand comprises a first coaxial cable including a first end and a secondend, and the first end of the first coaxial cable connects to the atleast one hardware interface to output the first electrical signal tothe dipole antenna, and receive the second electrical signal from theantenna.
 24. The monitoring system according to claim 23, wherein: thedipole antenna further includes a second coaxial cable including a thirdend and a fourth end, the first coaxial cable includes a first shieldand a first center conductor, the second coaxial cable includes a secondshield and a second center conductor, the first and second shields areelectrically connected, the first center conductor at the second end andthe second center conductor at the third and fourth ends terminatewithout any connection, a predetermined length of the first centerconductor is exposed at the second end and a predetermined length of thesecond center conductor is exposed at the fourth end, and the supportstructure maintains the exposed first and second conductorssubstantially parallel to each other.
 25. The monitoring systemaccording to claim 24, wherein: the plurality of coils include aplurality of first coils of the first coaxial cable and a plurality ofsecond coils of the second coaxial cable, and the support structuremaintains the plurality of first coils in a coiled configuration betweenthe first end and the second end and the plurality of second coils in acoiled configuration between the third end and the fourth end.
 26. Themonitoring system according to claim 25, wherein: the first coil regionincludes at least one coil of each of the plurality of first coils andthe plurality of second coils of the first antenna, and the second coilregion includes at least one coil of each of the plurality of firstcoils and the plurality of second coils of the second antenna.